Directly modulated laser with isolated modulated gain electrode for improved frequency modulation

Abstract

A DBR laser, such as a semiconductor DBR laser is disclosed having improved frequency modulation performance. The laser includes a split gain electrode and a tuning electrode. A modulating current encoding a data signal is injected into a first section of the gain electrode whereas a substantially DC bias voltage is imposed on a second section of the gain electrode positioned between the first gain electrode and the tuning electrode. The first and second gain electrodes are electrically isolated from each other and the tuning electrode by a large isolation resistance. In some embodiments, the isolation resistance is generated by forming the electrodes on a P+ layer and removing portions of the P+ layer between adjacent electrodes. Capacitors may couple to one or both of the second gain electrode and the tuning electrode.

1. A frequency modulated laser comprising:
 a modulated gain section, an un-modulated gain section, and a passive section; 
 a driving circuit coupled to the modulated and un-modulated gain sections, the driving circuit configured to impose a substantially constant tuning bias on the un-modulated gain section, and a modulating current encoding a data signal on the modulated gain section in order to generate a frequency modulated signal with a substantially flat frequency response; and 
 a tuning electrode and first and second gain electrodes; wherein the substrate is a semiconductor substrate having the tuning electrode positioned on the semiconductor substrate over the passive section, the first gain electrode positioned over the modulated gain section, and the second electrode positioned over the unmodulated gain section; and wherein the tuning electrode and first and second gain electrodes include metal and are bonded to a surface of the semiconductor substrate. 
 
     
     
       2. The laser of  claim 1 , further comprising a first capacitor having a first terminal coupled to the tuning electrode and a second terminal coupled to a reference voltage. 
     
     
       3. The laser of  claim 2 , further comprising a second capacitor having a first terminal coupled to the second gain electrode and a second terminal coupled to a reference voltage. 
     
     
       4. The laser of  claim 3 , wherein the first and second capacitor have capacitances substantially larger than 1/(ωR st ) where R st  is a serial resistance of the passive section and ω lies within the frequency modulation bandwidth of the frequency modulated signal. 
     
     
       5. The laser of  claim 1 , wherein the passive section includes a distributed Bragg reflector. 
     
     
       6. A method for modulating a laser comprising:
 imposing a tuning voltage on a tuning electrode positioned over a reflector portion of a semiconductor laser cavity; 
 imposing a bias voltage on a gain bias electrode positioned over a gain portion of the semiconductor laser cavity; and 
 imposing a modulating voltage on a gain modulating electrode positioned over the gain portion to generate a frequency modulated signal encoding a data signal, the gain bias electrode positioned between the tuning electrode and the gain modulating electrode, the gain bias electrode electrically isolated from the tuning electrode and the gain modulating electrode by an isolation resistance; 
 transmitting the frequency modulated signal through an optical spectrum reshaper operable to convert the frequency modulated signal into an at least partially amplitude modulated signal having amplitude modulation encoding the data signal. 
 
     
     
       7. The method of  claim 6 , wherein imposing the tuning voltage comprises imposing a substantially DC signal. 
     
     
       8. The method of  claim 6 , wherein imposing the bias voltage comprises imposing a substantially DC signal. 
     
     
       9. The method of  claim 6 , wherein the tuning voltage and the bias voltage have a frequency substantially less than the modulating voltage. 
     
     
       10. The method of  claim 6 , wherein imposing the modulating voltage on the gain electrode comprises imposing a bias signal and a data-encoding signal on the gain modulating electrode. 
     
     
       11. A fiber optic communication system comprising:
 a frequency modulated laser having a modulated gain section, an un-modulated gain section, and a passive section; 
 an optical spectrum reshaper (OSR) adapted to receive a frequency modulated signal from the frequency modulated laser and reshape the frequency modulated signal into a frequency and amplitude modulated signal; 
 a driving circuit coupled to the modulated and un-modulated gain sections, the driving circuit configured to impose a substantially constant tuning bias on un-modulated gain section, and a modulating current encoding a data signal on the modulated gain section in order to generate a frequency modulated signal; and 
 a tuning electrode and first and second gain electrodes; wherein the substrate is a semiconductor substrate having the tuning electrode positioned on the semiconductor substrate over the passive section, the first gain electrode positioned over the modulated gain section, and the second electrode positioned over the unmodulated gain section; and wherein the tuning electrode and first and second gain electrodes include metal and are bonded to a surface of the semiconductor substrate. 
 
     
     
       12. The fiber optic communication system of  claim 11 , wherein the surface includes a layer of p+ semiconductor material. 
     
     
       13. The fiber optic communication system of  claim 12 , wherein the layer of p+ semiconductor material defines a first gap extending therethrough between the first and second gain electrodes. 
     
     
       14. The fiber optic communication system of  claim 13 , wherein the layer of p+ semiconductor material defines a second gap extending therethrough between the second gain electrode and the tuning electrode. 
     
     
       15. The fiber optic communication system of  claim 11 , wherein an isolation resistance between the second gain electrode and the tuning electrode is greater than about 1 kOhm. 
     
     
       16. The fiber optic communication system of  claim 11 , wherein the isolation resistance between the second gain electrode and the tuning electrode greater than about 2 kOhm. 
     
     
       17. The fiber optic communication system of  claim 11 , wherein an isolation resistance between the second gain electrode and the tuning electrode and between the first and second gain electrodes is greater than about 1 kOhm. 
     
     
       18. The fiber optic communication system of  claim 17 , wherein the isolation resistance between the second gain electrode and the tuning electrode and between the first and second gain electrodes is greater than about 2 kOhm. 
     
     
       19. The fiber optic communication system of  claim 11 , further comprising a first capacitor having a first terminal coupled to the tuning electrode and a second terminal coupled to a reference voltage. 
     
     
       20. The fiber optic communication system of  claim 19 , further comprising a second capacitor having a first terminal coupled to the second gain electrode and a second terminal coupled to a reference voltage. 
     
     
       21. The fiber optic communication system of  claim 20 , wherein the first and second capacitor have capacitances substantially larger than 1/(ω R st ) where R st  is a serial resistance of the passive section and ω lies within the frequency modulation bandwidth of the frequency modulated signal. 
     
     
       22. The fiber optic communication system of  claim 11 , wherein the passive section includes a distributed Bragg reflector.